Capacitors used in electronic devices such as cellular phone and personal computer are demanded to have a small size, a large capacitance, a low ESR (equivalent series resistance) and high reliability. One of such capacitors is a solid electrolyte capacitor using a sintered body of valve-acting metal or electrically conducting oxide. This solid electrolyte capacitor is produced by connecting an anode lead terminal to a sintered body having fine pores in the inside, forming a dielectric oxide film layer throughout the sintered body surface including pore surfaces, sequentially stacking a semiconductor layer and an electrically conducting layer, and jacketing the obtained capacitor element.
The ESR value is roughly determined by the resistance of the semiconductor layer and the electrically conducting layer and therefore, various designs are being attempted. Particularly, in order to lower the leakage current value (hereinafter simply referred to as an “LC value”) caused by the dielectric oxide film, which is a most important factor for reliability, re-electrochemical formation is performed after forming a semiconductor layer, or the produced solid electrolyte capacitor is subjected to aging. Other than these, techniques of improving the semiconductor layer to obtain a low LC value have been proposed. For example, a technique of increasing the thickness of the semiconductor layer formed on the corner areas of the sintered body and thereby decreasing the short circuit failure of the produced capacitor (see, JP-A-13-143968 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)), and a technique of forming a semiconductor layer in a predetermined range (10 to 50 μm) on the surface layer of the sintered body and thereby obtaining a good value for both ESR and LC (see, JP-A-2003-188052) have been proposed.